At the present time, electronic timepieces which employ a stepping motor to drive time indicating means are in widespread use. There is an increasing demand for reduction of size, or increased duration of battery life for such timepieces, through reduction of the power consumption. Most of the power consumed in such a timepiece is utilized in driving the stepping motor, which in turn drives time indicating hands, and generally also a date display, through a gear train. In order to reduce power consumption to a minimum, it is obviously desirable to reduce the power applied to the stepping motor to the minimum level which is consistent with reliable operation. However, in the case of a timepiece which incorporates auxiliary time indicating means, such as date display, in addition to the time indicating hands, a difficulty arises in setting the drive power of the stepping motor to the minimum degree. This difficulty is due to the fact that the drive power required to be applied to the stepping motor when a date display is being actuated is considerably higher than that which is required when only the time indicating hands are being driven. Various methods have been proposed therefore for determining the load which is currently being applied to the stepping motor, and controlling the drive power applied to the stepping motor in accordance with the level of load. The stepping motor is generally driven by drive pulses (or drive pulse bursts, as explained hereinafter) of successively alternating polarity, with a period of one second between each drive pulse. Among the methods which have been proposed in the prior art have been those of providing an auxiliary winding adjacent to the drive coil of the stepping motor, and to detect the peak value of the voltage developed in this auxiliary winding, when a drive pulse is applied, to thereby detect a condition of increased load applied to the motor (U.S. Pat. No. 3,855,781 by Chihara et al). This has the disadvantage that a special type of stepping motor must be used. Another proposal has been to open-circuit the terminals of the stepping motor drive coil for a short time immediately after the cessation of each drive pulse, and to detect the voltage developed across the stepping motor drive coil at that time.
However, the latter method, as well as that of Chihara et al, and other prior art disclosures, has the disadvantage of instability of control, as will now be described. If we assume that the stepping motor load is increased above a predetermined level, then, with each of these prior art methods, this will be detected and a corresponding control voltage will be developed. This control voltage is used to cause a higher power drive pulse to be aplied to the stepping motor, as the next drive pulse after detection of the increased load. However, the effect of the increased power drive pulse will be to rotate the stepping motor rotor in a similar manner to that in which the rotor is rotated by a normal power drive pulse when under normal (i.e. relatively light) load. Thus, the detection means will fail to detect that an increased load is applied to the stepping motor, when detection is performed during or immediately subsequent to the increased power drive pulse, so that the next drive pulse after the increased power drive pulse will be a normal power drive pulse. It can be seen from this that a type of oscillatory instability, sometimes referred to as "hunting" is inherent in the prior art methods of controlling the drive power to an electronic timepiece stepping motor in dependence on the load applied to the motor. Because of this fundamental defect, such methods are of limited practical application for actually reducing the power consumption of electronic timepieces manufactured on a mass production basis. This is due to the fact that most of these prior art methods of detecting the load applied to the stepping motor are based on detection of a voltage (developed in the drive coil or in an auxiliary detection coil) whose value is determined by the angular velocity of the stepping motor rotor during or just after a drive pulse. When an increased power drive pulse is applied after an increased load condition has been detected, then this will result in the stepping motor rotor being accelerated to an angular velocity comparable to that which results from application of a normal power drive pulse under normal load. It is therefore extremely difficult, or impossible, for the detection circuit means to determine whether the stepping motor is operating under a condition of normal load, with a normal power drive pulse applied, or under a condition of increased load, with an increased power of drive pulse applied.
Another disadvantage is found in practice with the prior art methods in which the drive coil of the stepping motor is open-circuited for a short time just after the cessation of a drive pulse, and the level of voltage developed across the drive coil is detected. This is due to the fact that the voltage developed at this time is the sum of the electromotive force developed by the rotation of the stepping motor rotor through the magnetic field of the motor and the voltage developed by the collapse of the magnetic flux which was induced in the drive coil by the preceding pulse of drive current. It is therefore difficult to accurately predict the value of this composite voltage, and hence it is difficult to produce such detection means on a mass-production basis, without having to set-up the detection level for each individual electronic timepiece.
With the present invention, the above disadvantages of the prior art methods are eliminated. While the stepping motor is operating under normal load, detection of a voltage induced in the drive coil of the stepping motor is performed under a normal load detection status. When a load which is above a predetermined threshold level is detected in this normal load detection status, the operation then enters an increased load detection status, and an increased power drive pulse is applied as the next drive pulse to the motor. The increased load detection status is such that, so long as the increased load level applied to the stepping motor is maintained, the increased power drive pulses continue to be applied. When the load on the stepping motor falls below a predetermined level, then this is detected and the normal load detection status is then re-entered. Subsequently, normal power drive pulses are applied to the motor.
In this way, the control instability of the prior art methods is completely eliminated. In addition, with the present invention, detection of the drive coil voltage is performed only after effects induced by the drive current of the preceding drive pulse have been completely dissipated. This is ensured by performing detection of the drive coil voltage at an instant during one of several cycles of damped angular oscillation performed by the rotor of the stepping motor immediately after having been advanced by a drive pulse.